Shallow geothermal resources are less affected by atmospheric temperatures. In addition, it is maintained at a nearly constant temperature throughout the year. Therefore, when geothermal heat is used for heating and cooling, it is expected to be more energy efficient than using atmospheric air.
In Japan, the high cost of installing geothermal heat exchangers to use geothermal heat is an obstacle to its widespread use. For this reason, reducing the initial investment cost is very important for the use of geothermal heat. One cost-saving method is to install a geothermal heat exchanger in the core material of the retaining wall, which has been shown to reduce the construction cost by more than 50%.
In this study, a numerical analysis software was developed to clarify the performance and characteristics of a geothermal heat exchanger attached to an H-beam in a post-installation method using an earth retaining wall. Subsequently, the factors influencing the heat extraction and rejection performance of a single U-Tube, a double U-Tube connected in series, and two Downhole Coaxial Heat Exchangers connected in series were investigated. Furthermore, benchmarks for the heat extraction and rejection performance of different types of geothermal heat exchangers were made and compared using the “coefficient of heat extraction or rejection”, which is considered to be an index that allows quantitative evaluation of the heat extraction and rejection performance under different types of geothermal heat exchangers, installation environments, and operating conditions.
First, the calculation program was verified by comparing the numerical results with the experimental results of the thermal response test. Under the conditions defined in this study, increasing the flow rate of the heat transfer medium in all geothermal heat exchangers increases the heat extraction and rejection, but there is an upper limit. For the same temperature and flow rate of the injected heat transfer medium, the model with two Downhole Coaxial Heat Exchangers connected in series has the best heat extraction and rejection performance, while the model with a single U-tube has the worst. Furthermore, it was found that the installation of H-beams has the effect of expanding the area where the temperature of the formation changes due to heat extraction and rejection by the geothermal heat exchanger.
